The nine-month-long ALT program was conducted from February through November 1977 at the Dryden Flight Research Facility and demonstrated that the orbiter could fly in the atmosphere and land like an airplane, except without power-gliding flight.

The ground tests included taxi tests of the 747 shuttle carrier aircraft with the Enterprise mated atop the SCA to determine structural loads and responses and assess the mated capability in ground handling and control characteristics up to flight takeoff speed. The taxi tests also validated 747 steering and braking with the orbiter attached. A ground test of orbiter systems followed the unmanned captive tests. All orbiter systems were activated as they would be in atmospheric flight. This was the final preparation for the manned captive flight phase.

Five captive flights of the Enterprise mounted atop the SCA with the Enterprise unmanned and Enterprise's systems inert were conducted to assess the structural integrity and performance handling qualities of the mated craft.

Three manned captive flights that followed the five captive flights included an astronaut crew aboard the orbiter operating its flight control systems while the orbiter remained perched atop the SCA. These flights were designed to exercise and evaluate all systems in the flight environment in preparation for the orbiter release (free) flights. They included flutter tests of the mated craft at low and high speed, a separation trajectory test and a dress rehearsal for the first orbiter free flight.

In the five free flights the astronaut crew separated the spacecraft from the SCA and maneuvered to a landing at Edwards Air Force Base. In the first four such flights the landing was on a dry lake bed; in the fifth, the landing was on Edwards' main concrete runway under conditions simulating a return from space. The last two free flights were made without the tail cone, which is the spacecraft's configuration during an actual landing from Earth orbit. These flights verified the orbiter's pilot-guided approach and landing capability; demonstrated the orbiter's subsonic terminal area energy management autoland approach capability; and verified the orbiter's subsonic airworthiness, integrated system operation and selected subsystems in preparation for the first manned orbital flight. The flights demonstrated the orbiter's ability to approach and land safely with a minimum gross weight and using several center-of-gravity configurations.

For all of the captive flights and the first three free flights, the orbiter was outfitted with a tail cone covering its aft section to reduce aerodynamic drag and turbulence. The final two free flights were without the tail cone, and the three simulated space shuttle main engines and two orbital maneuvering system engines were exposed aerodynamically.

The final phase of the ALT program prepared the spacecraft for four ferry flights. Fluid systems were drained and purged, the tail cone was reinstalled, and elevon locks were installed. The forward attachment strut was replaced to lower the orbiter's cant from 6 to 3 degrees. This reduces drag to the mated vehicles during the ferry flights.

On March 13, 1978, the Enterprise was ferried atop the SCA to NASA's Marshall Space Flight Center, where it was mated with the external tank and solid rocket boosters and subjected to a series of vertical ground vibration tests. These tested the mated configuration's critical structural dynamic response modes, which were assessed against analytical math models used to design the various element interfaces.

In the April-October 1984 time period, Enterprise was ferried to Vandenberg Air Force Base in California and to Mobile, Ala. From there it was taken by barge to New Orleans, La., for the United States 1984 World's Fair.

In preparation for the first flight into space of OV-102 (Columbia), tens of thousands of hours of tests and simulations were expended at government and contractor facilities throughout the nation to qualify all the structures, flight equipment and computer program software.

The initial four flights of Columbia were launched from the Kennedy Space Center.and verified the design and operational capability of the space shuttle and all of the ground-based monitoring, communications and support systems. The first flight of Columbia was structured to minimize risks and complexity. The remaining test flights became progressively more complex and developed and demonstrated mission and payload capabilities. The spacecraft's performance was above and beyond expectations in all aspects. The spacecraft has demonstrated that it is a very stable platform for experiments in addition to providing a stable platform for the deployment of satellites.

The NASA SCA is a Boeing 747 (100 series) purchased from American Airlines on June 17, 1974. It was modified to ferry orbiters to and from various shuttle facilities and to transport and release OV-101 for the ALT program.

On Feb. 29, 1988, NASA announced plans to acquire a Boeing 747-100 to serve as a second SCA for the space transportation system. A letter contract was signed with Boeing Military Airplane Company, a division of the Boeing Company, Seattle, Wash., to reserve the aircraft for NASA use. The additional SCA will increase ferrying capability and eliminates a potential single-point failure in the space transportation system. This 747-100 will be modified to carry space shuttle orbiter's from landing sites to the processing facilities at the Kennedy Space Center.in Florida. The modifications will be made by Boeing at its manufacturing facilities in Wichita, Kan. This 747-100 is nearly identical to the original SCA and was selected to minimize costs associated with modifications and operation.

Modifications to the basic 747 aircraft included removal of interior equipment (passenger seats, galleys, etc.); changes to air conditioning ducts, electrical wiring and plumbing; installation of higher thrust engines (JT9D-7AHW) and the 747-200 series rudder ratio-charger; and alteration of the longitudinal trim system to permit 2 degrees more nose-down trim. Other changes included relocation and installation of antennas, addition of bulkheads and doublers in the fuselage main deck, addition of structural doublers and tip fins to the horizontal stabilizers to improve directional stability with the orbiter on top of the aircraft, and addition of one forward and two aft support assemblies for attachment of the orbiter. The modifications increased the basic weight of the aircraft by approximately 2,800 pounds.

The orbiter's mated location on the 747 was based on consideration of static stability and control, structural modifications, weight and performance. Center-of-gravity limits for the 747 with the orbiter mated were 15 percent of the 747's mean aerodynamic chord for the forward limit and 33 percent MAC for the aft limit. Longitudinal stability is similar to that of the basic 747; ballast must be added so that the center-of-gravity limits are not exceeded. The ballast is carried in standard 747 cargo containers in the forward cargo compartment. The mated configuration allows the 747's center of gravity to shift approximately 10 feet upward.

For the ferry flight configuration, the tail cone fairing is installed on the orbiter to decrease aerodynamic drag and buffet, and aerosurface control locks are added to the orbiter's elevons. The orbiter is unmanned and the orbiter systems inert. A bailout system also is installed in the 747.

Some modifications to the 747 SCA are removable. These include support struts for the orbiter, horizontal tip fans and associated cabling and umbilicals.